1. Field of the Invention
The present invention relates to an acoustic apparatus, utilizing a Helmholtz resonator, for radiating acoustic waves from both the resonator and a vibrator for driving the resonator and, more particularly, to a compact, wide-range acoustic apparatus which can perform lower bass sound reproduction and is free from noise or distortion.
2. Description of the Prior Art
As an acoustic apparatus utilizing a Helmholtz resonance, a phase-inversion (bass-reflex) speaker system is known. FIGS. 18A and 18B are respectively a perspective view and a sectional view showing an arrangement of the bass-reflex speaker system. In the speaker system shown in FIGS. 18A and 18B, a hole is formed in the front surface of a cabinet 1, a vibrator 4 consisting of a diaphragm 2 and a dynamic speaker 3 is mounted in the hole, and a resonance port 8 having a sound path 7 whose opening 6 is open to an external portion is formed therebelow In the bass-reflex speaker system according to the conventional basic design, a resonance frequency (antiresonance frequency) f.sub.OP defined by an air spring of the cabinet 1 and an air mass in the sound path 7 is set to be lower than a lowest resonance frequency f.sub.0 of the vibrator (speaker) when the vibrator is assembled in the bass-reflex cabinet. At a frequency higher than the antiresonance frequency f.sub.OP, the phase of the sound pressure from the rear surface of the diaphragm 2 is inverted at the sound path 7. Consequently, in front of the cabinet 1, a sound directly radiated from the front surface of the diaphragm 2 is in phase with a sound from the opening 6, thus increasing the sound pressure. As a result, according to an optimally designed bass-reflex speaker system, the frequency characteristics of the output sound pressure can be expanded to the resonance frequencies f.sub.0 of the vibrator or less. As indicated by an alternate long and two short dashed curve in FIG. 19, a uniform reproduction range can be widened as compared to an infinite plane baffle or closed baffle.
However, in the bass-reflex speaker system, open duct resonance occurs at the resonance port portion, and the resonant sound is radiated as noise or a distortion component of an acoustic wave.
In order to eliminate such distortion or noise, another acoustic apparatus wherein a small-diameter portion is formed in the central portion of a port to eliminate port resonance has been proposed (Japanese Utility Model Publication No. sho 54-35068). However, in this case, as the diameter of the small-diameter portion is decreased to enhance a filter effect, an acoustic resistance of the port is increased, and the Q value of the Helmholtz resonance is decreased. As a result, the behaviour of the speaker system approximates an operation of a closed type speaker system, and its frequency characteristics approximate those indicated by an alternate long and short dashed curve in FIG. 19. Therefore, bass-sound radiation power is decreased.
FIG. 20 shows an arrangement of an acoustic apparatus as described in a co-pending U.S. application Ser. No. 07/286,869 filed Dec. 19, 1988 and commonly assigned with the present application. In the system shown in FIG. 20, the resonance frequency f.sub.OP of a Helmholtz resonator is set to be still lower than that of a conventional bass-reflex speaker system, and a vibrator for driving the Helmholtz resonator is driven to cancel an air counteraction from the resonator when the resonator is driven, thus realizing a compact acoustic apparatus which can perform lower bass sound reproduction. FIG. 21 shows frequency characteristics of a sound pressure of the system shown in FIG. 20.
In FIG. 21, a solid curve represents frequency characteristics of an acoustic sound pressure resonantly radiated from the resonance port 8 of the resonator, and a broken curve represents frequency characteristics of an acoustic sound pressure directly radiated from the converter (speaker).
However, in the system shown in FIG. 20, since the length of the resonance port is increased in order to reduce the cabinet size and to decrease the resonance frequency of the Helmholtz resonator, an open duct resonance frequency of the resonance port is about 500 Hz, i.e., is lower than that of the conventional bass-reflex cabinet. Since the resonator is driven as described above, the Q value of the Helmholtz resonator is higher than that of the conventional one, and the amount of air passing through the resonance port is increased. For this reason, the frequency and level of an open duct resonant sound cannot be ignored, as indicated by peaks at frequencies f.sub.l and f.sub.2 in the characteristics curve of the resonator shown in FIG. 21.